The present invention generally relates to storage elements and more particularly, to a magnetooptical storage element in which recording, reproduction, erasure, etc. of data are performed by irradiating laser beams, etc. thereto.
Recently, magnetooptical storage elements have been greatly developed as optical memory elements enabling recording, reproduction and erasure of data. Particularly, a magnetooptical storage element in which a thin film made of an amorphous alloy containing rare-earth transition metals is used as a storage medium has such advantages that a recorded bit is not affected by the grain boundary and the film of the storage medium can be manufactured over a large area relatively easily, thereby attracting special attention. However, in the above described magnetooptical storage element in which the thin film made of the amorphous alloy containing the rare-earth transition metals is used as the storage medium, the photomagnetic effect (e.g. Kerr effect, Faraday effect) cannot be generally achieved to a full extent, thus resulting in an insufficient signal-to-noise ratio (S/N) of reproduced signals.
In order to eliminate such a problem, an element construction referred to as a "reflective film construction" has been conventionally employed in the magnetooptical storage elements as disclosed in, for example, Japanese Patent Laid-Open Publication No. 12428/1982 (Tokkaisho 57-12428). FIG. 1 shows a prior art magnetooptical storage element having the reflective film construction. The prior art magnetooptical storage element includes a transparent substrate 1, a transparent dielectric film 2 having a refractive index higher than that of the substrate 1, a thin film 3 made of an amorphous alloy containing rare-earth transition metals, a transparent dielectric film 4 and a metallic reflective film 5. In the known magnetooptical storage element of the above described construction, the thin film 3 has a sufficiently small thickness. Accordingly, when a laser beam L is incident upon the thin film 3, a portion of the laser beam L passes through the thin film 3. Therefore, both the Kerr effect, which is achieved by reflection of the laser beam L on the surface of the thin film 3, and the Faraday effect, which is achieved by transmission of the laser beam L through the thin film 3 upon reflection of the laser beam L on the reflective film 5 after the laser beam L has passed through the thin film 3, are exercised on the reproduced light. A Kerr rotational angle of the reproduced light superficially increases as large as several times that of a magnetooptical storage element subjected to only Kerr effect. Furthermore, the dielectric film 2 disposed on the thin film 3 also contributes to the increase of the Kerr rotational angle.
As one example of the known magnetooptical storage element in FIG. 1, the substrate 1 is formed by a glass plate, while the dielectric film 2 is made of SiO so as to have a thickness of 120 nm. Furthermore, the thin film 3 is made of Gd-Tb-Fe alloy so as to have a thickness of 15 nm, while the dielectric film 4 is made of SiO.sub.2 so as to have a thickness of 50 nm. Meanwhile, the reflective film 5 is made of Cu so as to have a thickness of 50 nm. In this example of the known magnetooptical storage element, the Kerr rotational angle increased to 1.75.degree. superficially.
Hereinbelow, a reason why the Kerr rotational angle increases extraordinarily in the magnetooptical storage element of the above described construction will be described. In the case where the laser beam L is irradiated onto the thin film 3 from the substrate 1 as shown in FIG. 1, reflection of the incident laser beam L is repeated in the dielectric film 2, so that interference of the repeatedly reflected laser beam L takes place and thus, the Kerr rotational angle increases superficially. At this time, as the refractive index of the dielectric film 2 is made larger, the dielectric film 2 further contributes to the increase of the Kerr rotational angle. Furthermore, an arrangement in which the reflective film 5 is disposed rearwardly of the thin film 3 also increases the Kerr rotational angle superficially. By interposing the dielectric film 4 between the thin film 3 and the reflective film 5, the Kerr rotational angle is further increased superficially.
Accordingly, a principle of this phenomenon will be described qualitatively, hereinbelow. It is assumed here that a reflective layer A is constituted by the dielectric film 4 and the reflective film 5. Thus, a first light ray incident upon the thin film 3 from the substrate 1 is reflected on the reflective layer A after passing through the thin film 3 and then, reversely passes through the thin film 3 again. Meanwhile, a second light ray incident upon the thin film 3 from the substrate 1 is directly reflected on the surface of the thin film 3 without passing through the thin film 3. Therefore, the first light ray which has passed through the thin film 3 reversely upon its reflection on the reflective layer A and the second light ray which has been reflected on the surface of the thin film 3 are combined with each other. At this time, since both the Kerr effect, which is achieved by reflection of the incident light L on the surface of the thin film 3, and the Faraday effect, which is achieved by transmission of the incident light L through the thin film 3, are produced in combination, the Kerr rotational angle is increased superficially.
In the magnetooptical storage element of such construction, it becomes highly important how the above described Faraday effect is added to the Kerr effect. Regarding the Faraday effect, if the thin film 3 is increased in thickness, the rotational angle can be increased. However, in this case, since the incident laser beam L is absorbed by the thin film 3, a desired purpose cannot be achieved. Therefore, a proper thickness of the thin film 3 ranges from approximately 10 to 50 nm and is determined based on the wave length of the laser beam L, the refractive index of the reflective layer A, etc. A requirement for the reflective layer A is that the reflective layer A should have a high refractive index.
Thus, in the magnetooptical storage element of the above described arrangement in which the dielectric film 2 is interposed between the substrate 1 and the thin film 3, and the reflective layer A is disposed rearwardly of the thin film 3, the Kerr rotational angle can be increased effectively. As is clear from the foregoing, a requirement for the reflective film 5 is that the reflective film 5 should have a high refractive index. In order to satisfy the requirement for the reflective film 5, the reflective film 5 is made of one of such materials as Au, Ag, Cu, Al, etc. However, these materials of the reflective film 5 undesirably lower the recording sensitivity of the storage medium due to their excellent thermal conductivity. Namely, in magnetooptical storage elements, recording of data is generally performed by inverting the orientation of magnetization through not only local heating of the storage medium by the use of laser beams but application of an auxiliary magnetic field to the storage medium from its outside. Thus, when the material of the reflective film has excellent thermal conductivity, heat given to the storage medium at the time of recording of the data is instantaneously diffused and thus, the temperature of the storage medium cannot be raised sufficiently to a required level.
Accordingly, the reflective film 5 is required to have not only a high refractive index but a low coefficient of thermal conductivity. The above described materials such as Al, Cu, AG and Au for the reflective film 5 are of high refractive indexes but have high coefficients of thermal conductivity. Therefore, the reflective film 5 made of one of the materials such as Al, Cu, Ag and Au is capable of improving quality of reproduced signals but disadvantageously lowers recording sensitivity of the storage medium.